A subscribers telephone is usually linked to the local telephone exchange by a telephone cable having two wires and usually known as the subscriber/two-wire line.
The local exchange is linked to other exchanges by a so-called four-wire line. Two of the four wires, (a signal line and a ground line) are used for transmitting telephone signals to other exchanges whilst the remaining two wires, (signal and ground) are used for receiving signals from other exchanges. In some cases a common ground line may be used thereby reducing the number of wires to three. The term four-wire is used herein to include such cases.
A BORSHT/SLIC circuit is located, in operation, in a telephone exchange and provides an interface between the four-wire line and the two-wire or subscriber line. The circuit has a four-wire interface including a receive terminal and transmit terminal and a two-wire interface to the subscriber line, formed by a pair of terminals.
A signal received on the receive terminal of the four-wire interface is applied by the circuit as a differential signal to the terminals of the two wire interface whilst a differential signal applied to the terminals of the two-wire interface, from the subscriber line, is fed to the transmit terminal of the four-wire interface.
Borsht and SLIC circuits must fulfill very stringent specifications for the rejection of common mode (longitudinal) signals which may appear on the subscriber line. Common mode signals are effectively identical currents flowing between each of the two wires of the two wire line and ground. Typically the common mode signals could be 50 Hz line current from the mains supply.
In addition to causing undesirable interference on the speech signals the common mode signals can be of large magnitude and can have a serious effect on electronic circuits coupled to the four-wire line.
To comply with the specifications, the gains in a number of different signal paths must be matched, or at least matched in appropriate pairs, to within the order of 0.1 or 0.2%.
It is generally not possible to build a SLIC or BORSHT having the required signal path matching except by adjustment after manufacture. To achieve this performance in a SLIC or BORSHT fabricated as an integrated circuit, recourse to laser trimming of thin film resistors deposited in the IC surface has generally benn envisaged.
It has also been proposed to use arrays of diffused resistors, with laser scribing to cut metal links therein, and combinations of rigorously symmetrical groups of diffused resistors which are assumed to yield the required matching provided the groups are large enough.
The above techniques of achieving the required signal path matching suffer serious drawbacks. The use of thin film resistors requires extra processing and this would at least increase manufacturing cost. Laser trimming or scribing is also expensive and causes potential reliability problems in high voltage circuits (BORSHT supply may be 150 v).
Diffused resistors change their values with stress due to bonding an IC to its package. Such stress could unbalance groups of resistors however distributed.
This invention seeks to provide a BORSHT/SLIC in which common mode signals may be reduced by adjustment of the gain of one or more signal paths wtihout recourse to laser trimming or scribing and to provide a method therefor.